The invention relates to a plant for producing cement clinker, having, viewed in the material flow direction, at least one heat exchanger for preheating raw meal, at least one downstream calcinator for calcining the raw meal, at least one rotary kiln for sintering the calcinated raw meal, at least one clinker cooler for cooling the sintered cement clinker, wherein a combustion device for difficult fuels, which have unpredictable ignition behavior, generally ignite poorly, or require an ignition aid in the form of strong heating and also initial pyrolysis, carbonizes, pyrolyzes, and/or combusts the difficult fuels, and a method corresponding thereto for the production of cement.
To produce cement clinker, a mixture of calciferous rock and siliceous rock is ground and subjected to a heat treatment, during which the lime contained in the raw meal is formally freed of carbon dioxide (CO2) and converted into burnt lime (CaO). In a further step, the raw meal, which is deacidified by the freeing from CO2, and which consists of the deacidified calciferous rock and of the siliceous rock, which is not yet changed here, is sintered in heat to form various calcium silicate phases.
The deacidification and also the sintering of raw meal are endothermic processes, which require thermal energy for the reaction thereof. The heat energy required for producing cement clinker is supplied to the process by combusting various fuels. High-quality fuels, which therefore also have a high caloric value, are supplied in a plant for producing cement clinker at important points to generate high temperatures, for example, a burner for generating high temperatures in a rotary kiln for sintering to form calcium silicate. At other points, at which generating high temperatures is less important than generating reductive conditions, fuels having a lower caloric value can also be used, for example, the combustion of dried household garbage, animal cadavers, shredded automobile tires, combustion of solvent wastes, or old paper. Other fuels available in larger quantities, which only ignite with difficulty, are petcoke, anthracite, high ash coals, and also alternative fuels made of industrial or community wastes. A substantial driver of costs of the variable costs in the production of cement is the price for fuels. The available fuels have different qualities with respect to combustibility, caloric value, moisture, flashpoint, and content of metals and other harmful materials, which cannot be destroyed by combustion.
So-called difficult fuels have unpredictable ignition behavior, typically ignite poorly or require an ignition aid in the form of strong heating and possibly initial pyrolysis. To employ these difficult fuels, a transition has therefore been made to pyrolyzing, carbonizing, and/or slowly combusting them with circulation and heating using a burner in a carbonizing furnace provided separately for this purpose.
In the case of carbonization of such difficult fuels, usually coarse, chunky waste materials, in a rotary kiln, however, it causes difficulties to provide uniform conditions for the thermal treatment, in particular to avoid the occurrence of temperature spikes, which can result in undesired solid baked-on deposits in the rotary kiln.
A cement clinker production line is known from EP 0764614 A1, having a separate shaft gasifier, which is installed adjacent to the calcining unit and is operated as a secondary reactor, in which coarse, chunky waste materials, in particular old tires, are to be gasified. In this case, the shaft gasifier is charged from above with the old tires and with a part of the tertiary air coming from the clinker cooler as the gasification agent. The product gas arising in the shaft gasifier is conducted as combustion gas into the calcinator, and the residual materials of the old tires gasified in the shaft gasifier are transported by a mechanical injection device into the rotary kiln intake shaft. The gasification process in the shaft gasifier can be subject to substantial variations, since the old tires stacked on one another in the shaft gasifier are not rearranged during their thermal treatment. An introduction of raw meal into the shaft gasifier, which thermally treats waste materials, in particular old tires, does not occur.
A cement clinker production line is known from WO 200109548, adjacent to the calcinator of which a separate reactor is installed, which is again filled from above with waste materials, which are to be combusted in the reactor using tertiary air from the clinker cooler. A partial stream of the cement raw meal is also introduced into the reactor likewise from above. The waste materials and the raw meal partial stream are to be deposited in the combustion reactor on a rotatable disk installed above the reactor floor, and the thermally treated solids are to be spun out by the rotation of the disk and spun into the calcinator or into the rotary kiln intake, and the combustion exhaust gas is also introduced into the calcinator. Mixing of the waste materials, particularly when they are provided in coarse, chunky form, also does not occur in the known combustion reactor, so that uniform combustion conditions and therefore uniform raw meal pre-calcination are not to be presumed.
In a plant for producing cement clinker with utilization of waste materials having high caloric value, from DE 3320670 A, DE 3411144 A, and DE 3520447 A, carbonizing or combusting the waste materials in a separate rotary kiln and using the carbonization gas/exhaust gas during the thermal raw meal treatment is also known. However, no cement raw meal is introduced into the carbonizing furnace/combustion furnace.
A method for firing raw materials such as cement raw meal, limestone, or other mineral raw materials is described in WO 2010032149. According to the method described therein, raw meal and a secondary fuel are placed separately from one another in the same rotary kiln.
A device and a method for producing cement clinker from cement raw material is disclosed in EP 1926959 A1, wherein dusts are heat-treated with the aid of combustion air and fuel in a dust burner reactor provided separately for this purpose, which is embodied as a rotary kiln. In this case, at least a part of the tertiary air is supplied as combustion air to the dust burner reactor and the dusts to be treated and the combustion air pass through the dust burner reactor in flow in the same direction. The location above the rotary kiln is provided as the location of the dust burner reactor for sintering the calcium silicate phases, the cement clinker.
A plant for producing cement having a further carbonizing furnace, which is used for carbonizing or combusting difficult waste materials, is disclosed in EP 1334954 B1. This carbonizing furnace is embodied as a rotary kiln and, according to the teaching of EP 1334954 B1, is to be arranged in parallel to the rotary kiln for sintering the calcium silicate phases, the cement clinker. According to the teaching of this document, arranging the rotary kiln for carbonizing and/or combusting the difficult fuels above the rotary kiln in the region of the calcinator is provided as a particularly advantageous location in the plant.
Finally, a carbonizing furnace embodied as a rotary kiln, which is not arranged above the rotary kiln for sintering the raw meal, but rather approximately at the same height as the rotary kiln, is disclosed in patent application DE 102012016361.4, wherein this carbonizing furnace is arranged as an extension of the rotary kiln for sintering. The pyrolysis gases exiting from this carbonizing furnace are used in the calcinator as a shared vertical duct for the exhaust gases of the rotary kiln and the carbonizing furnace for calcining the raw meal.
However, the above-mentioned carbonizing furnaces have the disadvantage that they place high mechanical demands on the structural foundation and require very substantial plant investments as moving parts.